Geology and Payette Lake


A geological map of Valley County, Idaho

A geological map of Valley County, Idaho

Geologists tend to have a strange time sense. Mostly, they live in the present, year-to-year, decade to decade, like the rest of us. But their profession also requires them to live in geologic time, in epochs spanning millions of years. A geologist would look at Payette Lake in both ways: a beautiful, blue and blessedly cold lake on a hot summer day, but also as a series of geologic events going back hundreds of millions of years. A geologist might see it something like this:

— An ocean shoreline, with sandy beaches and the proto-Pacific Ocean sending waves rolling in. Off shore, against the horizon, you might see a chain of large islands, much as you see Kodiak Island from the beach of the Alaska Peninsula. The inexorable forces of plate tectonics are moving those islands towards you, at about the rate your toe nails grow. Already, a big chunk of ocean floor has been shoved – subducted – under the North American Plate, where it slowly melts and forms a truly gigantic blob of molten rock. Some flows out on the surface, mostly further east, in the Challis Volcanics. Much of the rest very slowly congeals in place, forming the Idaho Batholith.

— Those volcanic islands offshore collide with the North American Plate, creating the Wallowa and Blue Mountains and moving the coastline 200 to 300 miles west. The very western edge of Valley County is part of that ancient island chain, docked against the old coast line. The very slow-motion collision is extraordinarily violent and raises up mountains that are still present along the western edge of Valley County. Former ocean floor can be seen along the tops of the mountains west of Payette Lake.

— The Idaho batholith is less dense than the rock it sits in, so it is gradually shoved upwards. Additional subducting ocean floor is shoved under it as well, lifting it upwards where water and ice erode rock above it and expose it to the world. The Idaho Batholith, the block of granite – technically, granodiorite – that sits in the middle of Idaho, was born. All that red color in the geologic map? That’s batholith. Deeply carved by water and ice.

— More volcanism associated with the Yellowstone Hotspot’s progress across southern Idaho deposits deep layers of volcanic rocks and ash across central Idaho.

— The Basin and Range “stretching” of the intermountain west raises West Mountain on the west side of what will become Long Valley, and Jughandle Mountain on the easterly side. As those mountains rise, the valley floor sinks, becoming a graben, a long trench. Erosion from the uplifting mountains on the sides of the graben fills the valley with eroded rock, sand and gravel – alluvium – in places to depths of 7,000 feet. The Basin and Range created Long Valley, roughly the pale yellow area on the geologic map.

— Most recently, just 15,000 years ago or so, Pleistocene glaciation carved the steep-walled north end of Long Valley, and excavated the deep trench that holds Payette Lake. The glaciation was massive enough to create decent-sized moraines, piles of soil and gravel. The moraines at the snout of the glacier – terminal moraines – dammed the outflow of Payette Lake, making it a bit deeper. A thick layer of glacial outwash was laid across the bottom of Long Valley.

A geologist looking at Payette Lake not only sees the water-skiers and boats. She also sees all that history, all of the geology that went before. Geologic time and human time running into each other.

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